Overload relays are current sensitive relays, normally used in conjunction with an electomechanical contactor, that may be used to disconnect power from equipment, for example, from a three-phase motor, when an overload condition exists.
In a typical installation, the contactor provides three contacts, one associated with each of the three phases of power, closed by an electromagnetically operated armature. The overload relay includes current sensing elements that are wired in series with the three phases passing through the contactor. In this way, the overload relay can monitor current flowing in the three phases through the contactor, and based on current magnitude and duration, may interrupt the current flow through the contactor armature circuit to open the contactor contacts when an overload occurs. For this purpose, the overload relay includes a set of latching contacts which can be used to control the contactor coil and/or provide a signal indicating an overload conditions.
Generally, an overload relay provides a different protective function than that of a circuit breaker which may also be wired in series with the contactor and overload relay.
Simple overload relays make use of mechanical elements, such as bimetallic strips for current sensing, communicating with contacts for providing a switched output. It is known, however, to construct overload relays from solid-state analog circuit components using current transformers for the current sensing element. Power for the solid-state analog overload relays is obtained through separate wiring or may be tapped from the secondary windings of the current transformers. The solid-state analog circuitry may be implemented in an application specific integrated circuit (ASIC) making the solid-state overload re-lay inexpensive and reliable.
It may be desirable to incorporate additional features into an overload relay, for example, to allow it to check for three-phase current imbalance, ground faults, or motor jam conditions. Remote operation and monitoring of the overload relay, for example, may also be desirable. These latter features may be implemented by providing dedicated wiring, to communicate, for example, a reset signal to the overload relay, or by providing the overload relay with a network connection, allowing serial digital data to pass between the overload relay and a separate controller.
When one or more of these additional functions is required, the analog circuitry of the overload relay is normally replaced with a microprocessor or microcontroller-based circuit. Measured current from the current transformers of the overload relay may be converted to digital values by an analog to digital converter and the base and supplemental protective functions are implemented in the microcontroller's firmware. The microcontroller may further implement a network interface allowing the overload relay to communicate with external sources for external control and readout of the overload relay function.
The use of a microcontroller in an overload relay substantially increases the cost of the overload relay, both because of the cost of the microcontroller but also because of the ancillary circuitry needed to support the microprocessor including power processing circuits, clock circuits, start-up circuits, memory and other interface circuits, and so-called “glue” logic circuits. Accordingly, additional overload relay functions are normally available only in feature rich, high priced overload relays. Mid-tier overload relays for users who need only a single additional feature, for example, represent a relatively low volume (fragmented by the number of different mid-tier products that are possible) making manufacturers reluctant to address this market.